I recently replaced my thermostat and the heater worked fine for a few months. Now the blower fan will not shut off when the room reaches the set temperature ( same problem I had before the new thermostat). I installed another known working thermostat and the problem remained so I know it is not a thermostat problem. What could be the problem?

Depending upon the brand and age of furnace it could just be a defective fan and limit control. On newer models it could be the circuit board if it controls the fan or relay.

Depending upon the brand and age of furnace it could just be a defective fan and limit control. On newer models it could be the circuit board if it controls the fan or relay.

My furnace is a 7 year old 80% effiencey Tempstar. How can I check weather it is a defective fan and/or limit cintrol?

Likely you have a stuck relay on a very expensive, hard to troubleshoot board. If you can see the board, you might try tapping the little square metal boxes on it. Hold the door switch closed while you do it. If tapping one of the little boxes causes the blower to stop. You have found the problem. Except for the price, replacing the board isn't too bad if you can note where the wires go on the old one and put them in the same place on the new one. Note, the board has 120 volts on it with the door switch held in. Watch what you touch.

On the limit switch. See if the furnace has a schematic showing if the switch should be open or closed.